[Suzerain of Sheol]

Well, 100 pounds is an exaggeration. More like 40. And seeing as Ser Jorah is an exiled knight from a different continent, I guess the Dothraki don't run into heavy armor a whole lot as they haven't adapted tactics against it.

And, actually, assuming a suit of plate was custom-made by a blacksmith who knew what he was doing, a knight should have been able to (and they did in fact) do cartwheels and backflips while armored.

I believe that whole "needing a crane to get on their horse" thing was for tourney armor, which was a lot bulkier and heavier to make jousting safer.